The present invention relates generally to methods for preventing air oxidation and thermal polymerization of furfural or furfural derivatives by introduction of anti-oxidant compounds. More specifically, the present invention relates to the use of compounds containing specific moieties for preventing or decreasing the oxidation of furfural compositions.
Furfural has many industrial applications. One of these includes its use to produce high quality petroleum lubricating oil by removing polycyclic aromatic compounds, sludges, and sulfur-, oxygen- and nitrogen-containing compounds. For example, polycyclic aromatic and oxygen compounds need to be removed from the lube oil fraction because they form polymers which plate out as varnish deposits on hot engine parts. The sulfur-, oxygen- and nitrogen-compounds in a lube oil fraction need to be removed because they can form acids which corrode types of alloys used in the manufacture of pumps and engines. Moreover, sludges must be removed from stocks of lube oil because they can reduce engine piston ring performance and may plug filters and oil lines. The impurities just described are removed from the lube oil feedstock by using furfural in a process referred to as furfural extraction.
In conventional processes using furfural extraction, the lube oil stock is mixed with the furfural in an extraction column. The lube oil/furfaral mixture is separated into two layers: one rich in furfural and containing the dissolved impurities (extract layer); the other rich in the desirable lube oil (raffinate layer). The main steps include: (a) extraction of the lube oil with furfural; (b) purification of the lube oil rich stream (raffinate); and (c) recovery of the furfural from the extract stream by a process which entails distillation after heating to about 450xc2x0 F. under vacuum with steam stripping.
Problems associated with the use of furfural in the aforementioned extraction processes include its tendency to oxidize when exposed to atmospheric or dissolved oxygen, and to polymerize when heated to elevated temperatures, even in the absence of oxygen. These reactions result in the formation of undesirable oxidation products such as formic acid, formyl acrylic acid or furoic acid which can, upon heating, further produce acidic polymers. The acidic oxidation products and the acidic polymers can cause a number of problems, including the corrosion of metal extraction process equipment. Moreover, the deposition of corrosion by-products and of acidic polymers on equipment surfaces can lead to a number of operational problems, including restricting flow of fluids.
In general, it is extremely difficult to prevent air from coming into contact with furfural during its manufacture, storage and use. However, economics dictates that the furfural be recovered and recycled following use and, for that reason, efforts to reduce the undesired oxidation of furfural have included the use of a number of anti-oxidant compounds aimed at preventing or retarding the oxidation of furfural. For example, U.S. Pat. No. 4,045,332 discloses the use of dialkyl anilines for preventing degradation of furfural during the process of solvent extraction. European patent applications EP 467,843 A2 and EP 467,844 A2 disclose the use of hindered phenolic compounds and amine compounds, respectively, as anti-oxidant compounds for use in furfural. Japanese 60090 295 A also discloses the use of phenolic compounds as a corrosion inhibitor in furfural/oil mixtures.
Examples of commercially available phenols used as antioxidants for furfural include di-butyl methyl phenol (BHT) and di-nitro-para-cresol (DNPC). In general, these phenolic anti-oxidants have the disadvantage that they are ineffective at maintaining the acid number low enough for commercial acceptability. The acid number, determined by potentiometric titration of furfural, is a measure of the acid generated during air oxidation of furfural; when comparing the performance of inhibitors of furfural oxidation, those with the lowest acid numbers relative to an untreated sample are the most effective inhibitors.
U.S. Pat. No. 5,453,519 discloses the use of hindered amines such as phenylenediamines or naphthalendiamines and their substituted derivatives as inhibitors of the oxidation, degradation or polymerization of furfural and other furan compounds. In particular, the patent discloses a method for introducing into the furfural these compounds while maintaining the acidity of the furfural below a pre-determined level by the addition of base. While phenylenediamines are alleged to be more effective at reducing acid formation during oxidation of furfural when compared with phenolic antioxidants, undesirable acidic polymers still may be generated even in the presence of such phenylenediamines. As described above, these polymers can cause fouling of equipment surfaces and restricted flow rates.
It would therefore be desirable to provide for other anti-oxidant compounds and compositions which can effectively reduce the acid number of furfural so as to significantly reduce its oxidation and degradation, while decreasing the formation of acidic polymers as compared to current methods and compositions.